1. Field of the Invention
This invention relates to an improved method for the determination of the corrosion rate of a metallic substance immersed in an electrolyte. It relates, more particularly, to a method employing the principles of electrochemistry and permits rapid, essentially instantaneous, measurements of the long-term corrosion rate.
The rate at which a metallic material is degraded at the surface in contact with a corrosive medium -- a chemically active fluid, moist earth, salt spray, or other -- is of considerable interest to the designers of metal structures and to those who provide protective coatings, inhibitors and other materials designed to slow down the rate of metal loss.
2. Discussion of the Prior Art
The traditional method most widely employed in the art of corrosion rate measurement is the exposure of a test coupon to the corrosive medium and the periodic removal of the coupon to determine the corrosion rate by weighing the remaining, unaffected metal. A variant on the above method employs an elongated test coupon of known initial electrical resistance and the periodic measurement of that quantity to determine the change in cross-sectional area and, hence, the weight loss.
These methods have inherent disadvantages: foremost the length of time required to obtain meaningful loss of material. Most corrosion rates are expressed in units of mils per year; that is, a surface exposed to the test conditions will lose so many thousandths of an inch per year through chemical attack. Because of the slowness of the reactions involved it takes months, and sometimes years, of exposure to obtain valid results and the experimental parameters must be closely controlled throughout such lengthy periods, often at great expense.
Another method of the art has been developed more recently and is based on fundamental concepts of corrosion as a result of electrochemical action. The basic expression of such concepts relies on Faraday's Law which, in a readily employed form, states that the weight of metal dissolved (W) is a function of the corroding current (I.sub.corr), the time of exposure to the current flow (t) and a fundamental property of the metal subjected to the corrosive environment (e). A constant (F) converts the above relationship to an equality, the value of F being well-known and corresponding to 96,500 coulombs of electrical charge. The equation is simply stated as: EQU W = 1/F .times. I.sub.corr .times. t .times. e
While Faraday's Law is universally accepted as valid and three of the four parameters on the right side are known, or readily defined, the application of the low to the determination of corrosion rates requires a knowledge of the corrosion current (I.sub.corr) under the applicable circumstances.
Because the corrosion current is a specific result of the particular corrosion cell which is created by the placement of a particular metallic body into the particular electrolyte under the particular environmental conditions for which the corrosion rate is sought, and because any attempt to measure it would alter the circumstances of the corrosion process, direct measurement of I.sub.corr is extremely difficult and would require elaborate procedures.
It is, however, possible to alter the circumstances of the corrosion process deliberately and to relate experimental measurements of currents so obtained back to an approximate value of the true I.sub.corr.
This method of the prior art is known as the Linear Polarization or Polarization Resistance method and is generally based on the Stern-Geary equation, as reported in the Journal of the Electrochemical Society, 104, 56 of 1957.
In the application of the polarization resistance method to the measurement of corrosion rates advantage is taken of the near linear relationship between a small impressed potential and the resulting increment in corrosion current. The metallic material whose behavior is to be studied is made part of a multi-electrode electrolytic cell whose electrolyte is identical to that in the proposed application. An external potential source is connected across the cell and the current flowing in the circuit, through the electrolyte and the electrode representing the metal under study, is measured. A new variable, the Polarization Resistance (R.sub.p) is then defined as the ratio of the small impressed potential (.DELTA.E.sub.app) to the resulting current (.DELTA.I.sub.app), with the value of the applied potential generally held at, or near, a value of 10 millivolts. Therefore: EQU R.sub.p = (.DELTA.E.sub.app /.DELTA.I.sub.app), .DELTA.E.sub.app .fwdarw. 0
When the above information is known, then the value of I.sub.corr can be derived from the relationship EQU I.sub.corr = K/R.sub.p
where the factor K is a function of the Tafel exponents defining the current flow under the circumstances where the well-known Tafel relationship applies. The Tafel Law states that the current flowing in such a circuit ought to be an exponential function of the applied voltage and it generally holds at high impressed potentials, above approximately 50 millivolts. These exponents are defined in terms of the potential increment required to effect a tenfold increase in the current flow and are different in value for the cases where the corrosion specimen is made the anode, or cathode, of the corrosion cell, respectively, by reversing the polarity of the impressed voltage.
The value of K relates to the anodic and cathodic Tafel exponents -- generally known as Tafel slopes in the are -- by the equality: ##EQU1## where B.sub.a and B.sub.c are the anodic and cathodic exponents, respectively, and 2.303 is a constant interrelating the value of natural logarithms and logarithms to the base 10; its presence is predicated on the aforementioned definitions of B.sub.a and B.sub.c in representing a tenfold increase in current.
In the techniques of the prior art, experimental determination is restricted to the value of R.sub.p and K is expressed in terms of assumed values of B.sub.a and B.sub.c, conventionally at 120 millivolts per decade. The advantage attendant on this procedure is the quickness with which experimental results are obtained, in a few minutes as opposed to months with the coupon immersion techniques; special corrosion cells and instrumentation have been developed to make the measurements simple and, in the terms of the art, substantially `instantaneous`.
Nevertheless, the assumptions regarding the values of the Tafel slopes introduce errors which can be, and often are, of considerable influence.
It is, therefore, a primary object of the invention to teach a novel method of corrosion rate determination which encompasses the experimental measurement of the potential/current relationship in the region obeying the exponential Tafel relationship and, thereby, increases the accuracy and reliability of substantially instantaneous corrosion rate measurements.
It is a further object of the invention to teach the use of such a method in corrosion cells incorporating a plurality of conducting electrodes, two to five in number.
It is yet another object of the invention to teach the construction and use of manual and automated apparatus for the performance of the method described above.